Radiation monitors are installed in a nuclear reactor facilities and the like for measuring radioactivity concentration of a radioactive rare gas in an exhaust gas. The exhaust gas is discharged from, for example, an exhaust stack that is a final discharge end of the facilities. The radiation monitor samples the exhaust gas of the exhaust stack and detects radiation released from the radioactive rare gas in a sampling gas. The radioactivity concentration is found by measuring a detection signal thereof (for example, see Patent Documents 1 to 5).
“Guidelines for measurement of released radioactive materials in light water nuclear power reactor facilities” are defined as domestic guidelines relating to the radiation monitor. According to the domestic guidelines pertaining to this release control, a detection unit is purged by air defecated by filters or the like and then background of the radiation is measured.
As for switching of flow paths of a sampling gas, ON/OFF switching of solenoid valves is performed in conjunction with a manipulation for selecting the flow paths for sampling and purge by switching. A state for selecting the flow paths is automatically indicated in order to prevent from forgetting manual switchback (see, for example, Patent Document 6). Normally, a flowmeter and pressure gauges are installed in gas flow paths of a radiation monitor of this kind. When abnormality of the gas flow paths is detected, a pump is automatically stopped and a system is automatically isolated by a solenoid valve (see, for example, see Patent Document 7).
A pump automatic restart circuit is provided for a pump stop due to instantaneous interruption of a supplied AC power source. The pump automatic restart circuit outputs a pump start signal with a set time interval when the power source is restored in a limited case where the power source is instantaneously interrupted within a set allowed time (see, for example, see Patent Document 8). Furthermore, normally, a sampling solenoid valve for selecting flow paths in an open state uses one that is operated to be opened in non-excitation/closed in excitation. In instantaneous power failure automatic restart, reliability of instantaneous power failure automatic restart is improved by securing the open state of a sampling flow path (see, for example, Patent Document 9).